Colicin S4

Colicin S4 is a type of Colicin, a bacteriocin made by E. coli which acts against other nearby E. coli to kill them by forming a pore in the membrane, leading to depolarisation of the membrane which kills the cell.

Synthesis and release
Colicin S4 is encoded on a plasmid operon, which also encodes its Colicin Immunity Protein, Csi to protect the cell making it from the cytotoxic activity. It does this by inserting into the membrane so that the pore cannot be formed. The colicin is produced in response to stresses such as nutritional deficiency to increase chances of survival when in competition with neighbouring cells. Also encoded in the same operon is the colicin release protein which helps the colicin leave the cell.

Mechanism of uptake
The receptor binding domain of colicin S4 binds to OmpW - an outer membrane porin protein on the outside of the target cell. This is constitutively expressed in the target cell, and parasitised by the colicin to gain entry. The translocation domain then recruits proteins from the Tol group of proteins to translocate across into the cell, through a mechanism as yet unidentified.

Killing Activities
Colicin S4 kills its target cell through its Pore Formation activity - it inserts into the membrane of the cell and causes depolarisation of the membrane. This is needed in the cell for ATP formation, so the cell is unable to make energy and therefore will die.